This invention relates to an improved method and circuit for controlling a cold cathode discharge vacuum gauge and, in particular, for extending the range thereof.
The description of the cold cathode discharge vacuum gauge was published by Penning in 1937. Such gauges consist of an anode and cathode within a vacuum enclosure having a tubulation to connect to the system whose pressure is to be measured. A potential of several thousand volts is applied between the electrodes, and a magnetic field is provided such that the magnetic field is perpendicular to the electric field of the electrodes. Electrons are trapped in the crossed electric and magnetic fields. Collisions of the electrons with residual gas molecules produce ions, which are collected by the cathode. With this arrangement, a pressure dependent current is found in the anode--cathode circuit which obeys the relationship EQU I.sub.gauge =K p.sup.n
over a pressure range of approximately 10.sup.-8 to 10.sup.-4 Torr.
In this equation, I.sub.gauge is the observed gauge current, K is a constant, P is the pressure, and n is a constant whose usual value is between 1.00 and 1.15 depending upon the specific gauge. FIG. 1 shows a typical current versus pressure characteristic for a gauge of the inverted magnetron type operated with an anode voltage of 4.00 kV, and with a one megohm in series with the high voltage power supply to anode lead. The current departs from the power law for pressure below 10.sup.-8 Torr, but obeys it accurately at higher pressures. Historically, most cold cathode ionization gauges simply use this current--pressure relationship to determine the pressure.
As may be seen from the curve of FIG. 1, the tube input current and thus the power become appreciable at pressures greater than 10.sup.-4 Torr. At 10.sup.-4 Torr the current is 4.times.10.sup.-4 A, and the power 1.6 W. The effect of operation at high currents is to accelerate damage to the tube. Ion sputtering leads to the formation of conducting films on the high voltage feedthrough insulators and the support insulators for the electrodes. Leakage currents in these films mask the small ion currents at low pressures. Allowing the current to increase to larger values also means that heavier and more expensive high voltage supplies are required.
It is desirable, however, to be able to operate a gauge over the widest possible range. Otto Klemperer in British Patent No. 555,134 noted that at high pressures it was possible to operate a cold cathode gauge in another manner. He stated that the anode voltage needed to maintain a fixed small current was a measure of pressure. In this mode of operation the power input presents problems. The power supply must function at constant voltage at low pressures, and constant current at high pressures. The measured signal is current in one case and voltage in the other. The changeover in operation would almost inevitably produce a discontinuity and therefore an inaccuracy, in the pressure displayed.